As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries, which exhibit high energy density and discharge voltage, has been underway and such lithium secondary batteries are commercially available and widely used. Lithium secondary batteries have long electrode lifespan and excellent high-speed charge and discharge efficiency and thus are used most widely.
In general, a lithium secondary battery has a structure in which an electrode assembly, which includes: a cathode including a lithium transition metal oxide as an electrode active material; an anode including a carbon-based active material; and a polyolefin-based porous separator disposed between the cathode and the anode, is impregnated with a lithium salt-containing non-aqueous electrolyte, such as LiPF6 or the like.
In this regard, a lithium cobalt-based oxide, a lithium manganese-based oxide, a lithium nickel-based oxide, a lithium composite oxide, and the like are mainly used as cathode active materials, and a carbon-based material is mainly used as an anode active material. Lithium ions of a cathode active material are deintercalated and then intercalated into a carbon layer of an anode during charge, the lithium ions of the carbon layer are deintercalated and then intercalated into the cathode active material during discharge, and a non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and the cathode. Such lithium secondary batteries basically require stability within operating voltage ranges of a battery and the ability to transfer ions at a sufficiently rapid rate.
However, in secondary batteries using a fluorine (F)-containing electrolyte and a carbon material as an anode active material, as a charge and discharge process progresses, metal components of a cathode active material are eluted into an electrolyte and lithium is deposited onto a surface of a carbon material and, accordingly, the electrolyte decomposes at the carbon material. Such deposition of metal components and decomposition of an electrolyte more severely occur when a secondary battery is stored at high temperature, which results in reduction in battery remaining capacity and recovery capacity.
Meanwhile, a lithium transition metal oxide used as a cathode active material has low electrical conductivity, and reaction between the lithium transition metal oxide and an electrolyte is accelerated at high temperature, generating a by-product that increases resistance of a cathode, which results in drastic reduction in storage lifespan at high temperature.
To address these problems of a cathode and an anode, the related art discloses a technology for coating or treating a surface of a cathode or anode active material with a predetermined material.
For example, Japanese Patent Application Laid-open No. 2000-12026 discloses a method of coating an oxide of a metal such as Ni, Co, Cu, Mo, W, or the like on a surface of a carbon-based anode active material. In addition, as a method of coating a cathode active material with a conductive material to reduce resistance of a contact interface between the cathode active material and an electrolyte or a by-product generated at high temperature, a method of coating a cathode active material with a conductive polymer is known.
In addition, Korean Patent Application Publication No. 2003-0088247 discloses a method of preparing a cathode active material for a lithium secondary battery, including: (a) surface-treating a metal-containing source by adding the metal-containing source to a doping element-containing coating solution (wherein the metal-containing source is a material containing a metal selected from the group consisting of cobalt, manganese, nickel, and combinations thereof and excluding lithium); (b) preparing an active material precursor by drying the surface-treated metal-containing source; and (c) mixing the active material precursor and the lithium-containing source and heat-treating the resulting mixture.
However, a water-soluble material cannot be used in coating of a calcined electrode active material and, when an oxide is used, it is difficult to smoothly coat an already synthesized material with the oxide.
The related art discloses coating with OH groups, but it is difficult to form a uniform film using this method, and only restrictive materials in accordance with pH and the like may be used and thus there is limitation in coating composition.
Therefore, there is a high need to develop a technology that may fundamentally address these problems.